A method to calculate a mass flow rate of rarefied gas through a long tube of variable radius subject to arbitrary drops of pressure and temperature is proposed. The method is based on the results obtained previously on the basis of the kinetic equation and it requires very modest computational efforts. As a demonstration of its application some examples of isothermal and nonisothermal flows are given. The thermomolecular pressure difference for a variable radius tube is also considered. © 2005 American Vacuum Society.